1. Field of the Invention
This invention relates to preventing reductive corrosion of indium tin oxide (ITO) in the presence of aluminum in alkaline or basic solutions or solvents, and more particularly, to a method and apparatus for maintaining the quality of electrical and optical properties of an ITO layer during fabrication of a flat panel display.
2. Description of the Related Art
Optically transparent and electrically conductive materials such as indium tin oxide (ITO) find utility in flat panel display (FPD) industries such as field emission displays (FEDs), liquid crystal displays (LCDs), and organic light emitting devices (OLEDs), as well as in solar cells. Surface and bulk characteristics are imperative to the quality of electrical and optical properties of these and other optically transparent and electrically conductive films. It is therefore very important to ensure that such films exhibit the desired surface and bulk properties such that the desired degree of transmission of visible light and electrical properties are obtained.
Devices incorporating ITO often use an aluminum layer deposited over the ITO. For instance, in an FED device where the faceplate is connected to the baseplate using spacers, aluminum is often deposited over the ITO layer in the faceplate to establish sites for the bonding of misaligned spacers. More particularly, an aluminum layer is formed over the ITO layer, the aluminum layer having wells extending therein to the surface of the ITO layer. Bond pads are provided within these wells against the ITO layer at the desired spacer locations. Then, when an array of spacers is brought against the faceplate for anodic bonding, desired spacers contained in the array will bond to the bond pads, while other, misaligned spacers will bond to the aluminum layer. After bonding is complete, the aluminum layer with the misaligned spacers bonded thereto can be removed to leave the desired spacer configuration in the FED.
A problem with using aluminum with ITO in the above and other applications is that ITO is susceptible to corrosion in the presence of aluminum. Atomic and/or ionic diffusion occurs through the aluminum to the ITO during processes such as anodic bonding, thermal cycling, thermal diffusion processes, low energy ion implantation processes, and processes which include electric and/or magnetic fields. ITO is especially susceptible to corrosion in the presence of aluminum when exposed to alkaline or basic solutions or solvents. See J. E. A. M. van den Meerakker and W. R. ter Veen, J. Electrochem. Soc., vol. 139, no. 2, 385 (1992). Corrosion of ITO in alkaline solutions produces SnO32xe2x88x92, which dissolves in the solution, and In metal, which forms grains at the surface. This causes a gray opaque appearance and a disconnection between the ITO and aluminum. Corrosion of the ITO can prove fatal in devices such as flat panel displays by reducing or eliminating the electrical conductivity and optical transparency of the ITO material. This corrosion can also cause delamination of the aluminum layer from the ITO. Redeposition of corrosion byproducts onto the substrate leads to additional defects, e.g., particle defects.
This problem becomes especially pronounced when the fabrication of the display or other devices includes a photolithography step. For instance, during the development stage of photolithography, diffusion of the developer, such as TMAH or other basic developers, through the aluminum causes reductive corrosion of ITO. Furthermore, stripping of the photoresist involves the use of a solvent-based wet process that may also cause corrosion. This is especially true when the pH of the solvent stripper is greater than seven and the ratio of aluminum to ITO surface area exposed to the solvent is high.
Accordingly, what is needed is a method and apparatus to prevent the corrosion of ITO when in contact with aluminum during photolithography or when otherwise exposed to corrosion-inducing solutions.
Briefly stated, the needs addressed above are satisfied by forming a protective layer such as chromium, chrome alloys, nickel or cobalt as a cap over the aluminum film to protect the underlying ITO layer from corrosion. The presence of the protective layer during fabrication processes such as photolithography prevents diffusion of solutions through the aluminum into the ITO. This protective layer is especially effective during the development and resist stripping stages of photolithography which use solutions or solvents that would otherwise cause reductive corrosion of ITO in contact with aluminum. The methods and apparatus described herein are particularly advantageous for the fabrication of flat panel displays such as field emission devices and other display devices, because ITO is often used in such devices in contact with aluminum while exposed to corrosion-inducing media.
In accordance with one aspect of the present invention, a method is provided for preventing corrosion of a structure having a tin oxide layer in contact with an aluminum layer while exposed to a solution. A protective layer is formed on the aluminum layer, thereby exposing the protective layer to the solution while reducing the surface area of the aluminum layer exposed to the solution. This protective layer inhibits diffusion of the solution through the aluminum layer into the tin oxide layer. In one embodiment; the tin oxide layer is made of indium tin oxide and the protective layer is made of a material selected from the group consisting of chromium, chrome alloys, nickel and cobalt. After the protective layer is no longer exposed to the solution, the protective layer is removed.
In accordance with another aspect of the present invention, a display device structure comprises a substrate, an electrically conductive and optically transparent layer over the substrate, an aluminum layer over the electrically conductive and optically transparent layer, and a protective layer over the aluminum layer. In one embodiment, the electrically conductive and optically transparent layer is made of indium tin oxide. In another embodiment, a barrier layer is provided between the layer of electrically conductive and optically transparent material and the aluminum layer.
In accordance with another aspect of the present invention, a method of manufacturing a display device structure is provided. An indium tin oxide layer is formed over a substrate. An aluminum layer is formed over the indium tin oxide layer. A protective layer is formed over the aluminum layer. In one embodiment, the display device structure is exposed to an indium tin oxide-corrosive medium, and the protective layer acts as a barrier to the corrosive medium.
In accordance with another aspect of the present invention, a method of patterning an aluminum layer over a tin oxide layer is provided. The method comprises forming an aluminum layer over the tin oxide layer, forming a protective layer over the aluminum layer, and forming a photoresist layer over the protective layer. Regions of the photoresist layer are selectively exposed to ultraviolet light, and these regions of the photoresist layer are developed to expose selected regions of the protective layer. The exposed selected regions of the protective layer are removed to expose selected regions of the aluminum layer. The exposed selected regions of the aluminum layer are removed, and the photoresist layer is stripped from the protective layer. The protective layer is removed to leave the desired device structure.